I was able to make it work by:
1. Unpacking the data.acd in CM
2. Editing 1st.rto, 2nd.rt ... final.rto to reflect the ratios you want, and the gear pairs that make those ratios. (this is the work. I had to use a spreadsheet, cos gear ratios are hard. you're guessing which two integers will produce an accurate to 4 decimals ratio. I tried to stay realistic with how many teeth you could actually fit in a gear and have it fit in the gearbox casing.)
3. Editing drivetrain.ini in the [Gears] section, with the data you created in the .rto files.
4. Editing setup.ini to ensure that each [GEAR x] references the correct .rto (this may not be needed. I had to)
Some notes:
1. In each .rto, I only offer the single gear pair that I dreamt up to achieve the ratio required. In the setup screen in game, you now no longer have the ability to fine-tune ratios (there's only one for each gear). You could, by dreaming up your own alternative gear pairs in each .rto add this ability back.
2. My edited files are here. Place in the unpacked data folder, and rename data.acd. But first.... make a backup, eh?
mega.nz
3. This feels like a good change, I was immediately faster around Le Mans with this ratio set. YMMV.
4. Holy cow, the ratios Kuno choose are wildly different than those you got. ???
Have fun.